1. Field of the Invention
The invention relates to a process for producing a thermoplastic elastomer compound (TPE) in the form of a mixture of one or a plurality of thermoplastics and one or a plurality of crosslinked rubbers, such a mixture being produced from
one or a plurality of thermoplastics in an amount of from 10 to 90% by weight; PA1 one or a plurality of rubbers; PA1 a crosslinking activator; PA1 crosslinking agents, as well as PA1 other additives, if required; PA1 with the use of a roll system comprising a feeding opening of a conveying screw feeding the first roll element, as well as additional roll elements with point for adding other materials within a transition area of the roll elements, whereby each roll element has a main spindle (central spindle) and a plurality of rotating spindles, and whereby, furthermore, the components of the mixture are fed into the roll system via the feeding opening and the points for adding other materials. PA1 The thermoplastic/thermoplastics and/or non-crosslinked rubber/rubbers are fed into the roll system via the feeding opening in the absence of the crosslinking agent, and are subsequently melted open and dispersed, with formation of a homogeneous mixture. PA1 The homogeneous mixture is transported in the roll elements while the crosslinking agent is simultaneously added in a transition zone of the roll elements; and PA1 the mixture provided with the crosslinking agent is dispersed and homogenized as the vulcanization is being carried out simultaneously at expansion rates of 100 to 1500 s.sup.-1, especially 500 to 1500 s.sup.-1, with formation of the thermoplastic elastomer compound. PA1 The thermoplastic/thermoplastics and/or the crosslinked rubber/rubbers and the crosslinking agent are jointly fed into the roll system via the feed opening, and are subsequently melted open and dispersed; and PA1 the mixture is dispersed and homogenized at expansion rates of 100 to 1500 s.sup.-1, especially 500 to 1500 s.sup.-1, as vulcanization takes place at the same time, with formation of the thermoplastic elastomer material.
2. The Prior Art
The elastomer compound of the type specified above is a blend consisting of a thermoplastic material and a crosslinked rubber. Crosslinking of the rubber is carried out by dynamic vulcanization. The term "dynamic vulcaniztion" is understood to be a process in which the thermoplastic material, the rubber and the crosslinking system are masticated (melted open and dispersed) as the rubber is being crosslinked. Examples of dynamically crosslinked thermoplastic elastomers and the process of dynamic vulcanization are described in U.S. Pat. Nos. 4,130,535 and 4,311,628. German laid-open patent specification 26 32 654 describes a blend consisting of a thermoplastic polyolefin, an EPDM rubber and any desired crosslinking system known from U.S. Pat. No. 3,806,558. The rubber is vulcanized to such an extent that it contains no more than about three percent of rubber extractable in cyclohexane at 23.degree. C. Furthermore GB-A-2 007 683 describes a thermoplastic elastomer containing a thermoplastic crystalline polyolefin resin and vulcanized EPDM. Crosslinking of the rubber is carried out in a phenolic resin. The degree of crosslinking so obtained exceeds 97%. It is pointed out in EP 0 107 635 B2 that the blending methods employed conventionally up until that time for producing the dynamically vulcanized thermoplastic elastomers are not suitable for producing soft mixtures with good extrudability. Said document describes a one-stage process carried out on a double-screw extruder with screws rotating in the same sense, which, at high shearing rates of &gt;2000 s.sup.-1 and with a dwelling time of &lt;2 minutes permits the manufacture of soft thermoplastic elastomers with good extrudability. Masticating of the rubber and other components takes place in the first third of the double-screw extruder. Dynamic vulcanizing takes place in the last two thirds. A similar process for the production of a dynamically vulcanized thermoplastic elastomer is described in EP 0 072 203 A2, where in a one-stage process carried out on a double-screw extruder with screws revolving in the same sense, a partially crosslinked thermoplastic elastomer is produced from a peroxidically crosslinkable olefinic copolymer rubber, a polyolefin resin not degradable by peroxide, and an organic peroxide. The components of the mixture are directly admitted into the double-screw extruder. Furthermore, EP 0 547 7843 A1 describes a one-stage process for producing a dynamically vulcanized thermoplastic elastomer, whereby an olefinic copolymer rubber and a thermoplastic polyolefin are here directly fed into a blending extruder. An organic peroxide is added farther downstream in the direction of flow in the plasticizing unit. The shear rate of the dynamic vulcanization is &gt;500 s.sup.-1. Single-screw, double-screw or multiple-screw extruders can be employed for the procedure described in said document.
The mixing methods known heretofore for producing a dynamically crosslinked thermoplastic elastomer are afflicted with the following problems:
It is not possible with a kneader (inner mixer) to achieve adequate dispersion and homogenization in the course of dynamic vulcanization of the elastomer phase. High shear rates can be achieved only inadequately or not at all due to the circumstances described in EP 0 107 635 B2. The production of soft blends with good extrudability is not possible with this process.
A double-screw extruder with screws revolving in the same sense is capable of assuring sufficient dispersion and homogenization. The production of soft, well-extrudable thermoplastic elastomers with a dynamically crosslinked elastomer phase is admittedly possible with this procedure; however,high shear rates of &gt;2000 s.sup.-1 are required. Such high shear rates and the mechanical energy introduced thereby in the plastic material are converted into heat, which leads to a considerable rise of the mean temperature of the melt in general and, in detail, to temperature and viscosity inhomogeneities because of the poor temperature conduction values. The high admission of energy simultaneously combined with rising mass temperatures leads to increased material stress, or even degradation of the material. Because of the surface conditions it is not possible with the known twin-screw extruder with screws revolving in the same sense to discharge again the admitted energy by means of the cooling system available, and to permit careful dispersion and homogenization in the course of dynamic vulcanization of the rubber phase. Fitting the pair of screws with conveying, mixing and kneading elements thus always constitutes a compromise between the admission of energy and the increase in mass temperature of the melt.